OEM E90 wheel specs are 19x8.5 ET29, 19x9.5 ET23; so they're not OEM if they're +45 offset.
You should have no issues with 19x9 ET45 and a 255.

My setup is:
18x8 ET35 225/40
18x9 ET32 245/35 (rolled guards)

EDIT:

Offset depends on wheel width, normal staggered fitment that most people with aftermarket wheels run on an E46 is 8.5" Front and 9.5" rear, with offsets around 35 and 40-45 respectively. Just depends how aggressive you want to go.

Cheers for the info man, looks like I'll have to look into new rear tyres then